In order to couple a signal between different devices (e.g. integrated circuits) or circuits which operate with different supply voltages, it is known to perform a level shifting of the signal, this being achieved by a level shifting circuit or interface circuit between the devices. The level shifting circuit can be regarded as an output circuit of the device supplying the signal, as an input circuit or buffer of the device receiving the signal, or as a separate interface circuit between the two devices. Typically, the extent of the level shifting will depend upon the differences between the supply voltages of the devices, and the nominal signal levels which typically also depend on the supply voltages.
For example, especially for handling signals at very high frequencies (e.g. 5 GHz), integrated circuit devices using BCMOS technology may be used which operate with nominal supply voltages of either +1.8 volts and -5 volts or 0 (zero) volts and -5 volts, in each case typically with a tolerance of .+-.10%. In the former case, a signal may have high and low voltage levels of 1.8 and 1.4 volts respectively, with a signal voltage swing of 0.4 volts; under worst case conditions, this signal voltage swing may be as little as 0.2 volts. In the latter case, a signal may have high and low voltage levels of 0 and -0.8 volts respectively, with a signal voltage swing of 0.8 volts; under worst case conditions, this signal voltage swing may be as much as 1.0 volts.
From these figures, it can be seen that an integrated circuit device with a differential supply voltage of about 7.5 volts (1.8 +5 volts, plus 10%) may produce a signal voltage swing of only about 0.2 volts, and a device with a differential supply voltage of 4.5 volts (5 volts minus 10%) may produce a signal voltage swing of 1 volt. Thus there is considerable variability of both the signal voltage swing and the differential supply voltages which may be present.
A considerable problem is presented in providing an appropriate interface circuit, for example as an input buffer to a device operating with the nominal 0 and -5 volt supply voltages, which can properly handle signals from a preceding device which may correspond to either of the two cases discussed above. Such an interface or signal level shifting circuit is desired to handle, relatively arbitrarily, a signal with either set of supply voltages and signal voltage swings, because a preceding device producing the signal may fit either of these two cases.
This problem is exacerbated by constraints which are imposed by the BCMOS technology and the desired functioning of the devices. For example, BCMOS technology may impose a constraint that a maximum voltage difference between any two terminals of any transistor or other active device be no more than 3.6 volts for devices operating at signal frequencies, or about 4.5 volts for devices used for biassing. In addition, the devices are desired to have substantially constant biassing conditions under the different possible signal handling conditions, to ensure that performance (e.g. bandwidth) is not dependent upon the nature of the supplied signal. In addition, low power dissipation is desired.
A need exists to provide a signal level shifting circuit which enables these desires to be satisfied.